To Mach or not to Mach!

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Guest rmce

OK a couple of questions.The easy one is how do I place my picture on the Splash Screen of Fs2KPro?Harder one is the old chestnut about Mach at altitude.I have just returned from Toronto to Amsterdam on a KLM 747-400.It was a night flight and cabin info showed a speed of 616 at 37K.Weather was clear and wonderful views of Montreal and Ottawa were seen.KLM Captain announced that he has picked up a 100 mph tail wind and sure enough speed increased to around 650, so flight was 6.5 hours coming in half hour early.So, how close were we to Mach 1?What happens if 480 tons of 747-400 (half to three quarters full) reaches Mach 1. Does it pass the magic number?My fellow traveller, knowing my interest in FS stuff asked this. I was dammed if I could answer!! RmceGlasgow

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My first thoughts here. It doesn't matter what GS (ground speed) you're having when deciding what Mach you have. It's all about IAS (Indicated AirSpeed). You where still travelling at the same Mach number even if you increased your GS. I may be totally out of line here so please tell me if I'm wrong. See this link for a further in depth explanantion:http://www.womanpilot.com/past%20issue%20p...00/airspeed.htmHope it clears :-)

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Guest rmce

Mats,Thanks for the reply.Excellent article you posted link to.Now, can I understand the maths? No problem. I cannot!OK so IAS is I know different from actual airspeed.I suppose 650 knots is fast enough be it indicated or actual. I suppose cabin readouts are indicated. Or are they?rmce

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Thanks sir!>I suppose 650 knots is fast enough be it indicated or actual. I>suppose cabin readouts are indicated. Or are they?Actually that was one question that was rasied when I read your post. What speed are we actually reading in the cabin. I would think it is GS cause it's the speed most easy to relate to. Any aviator here to enlighten us?>Now, can I understand the maths? >No problem. >I cannot!Well, Rmce, neither can I at a glance despite my BSc :-) But I hope you get the outlines of how to determine super-, trans-, and subsonic speeds. As stated in the article most air liners do have a general limit of 0.85 Mach. However I think the B747 has a better record regarding that maxima. regards

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>What speed are we actually reading in the cabin. >I would think it is GS cause it's the speed most easy to >relate to. Any aviator here to enlighten us? You mean speed displayed for passengers ? There are only 2 possibilities. It is either so called TAS (true air speed) or Ground Speed. With no winds they are identical. But which it is exactly - I have no clue. If I had to guess I would say TAS.Michael J.[link:jdtllc.com]http://jdtllc.com/images/RCsupporter.jpg

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Guest rmce

micheal,Thanks for reply.<>Indeed. Or to put a reality test to it, flying at 650KPH in 480 tons of strings and sealing wax, at 7 miles high, still strains credulity beyond science.I could look out over the starbord wing and enjoy this massive piece of metal fluctuate and flap around. I could explain that this is a good thing this flapping, and had nothing to do with getting us and keeping us at 7 miles high+.Of course in reality, one does still wonder just how the hell it keeps those massive engines attached! Just the kinda helpful thought one has as one drops off to sleep as the dim the lights and KLM wish you "good night and see you in the morning, around about Ireland" in about three hours time! Were they, KLM, going to sleep to?rmce

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Guest

hi guys,the only answer is that the displayed speed (in knots) in the cabin is the groundspeed: a mach number refers to an IAS and is almost constant in cruise (depends whether aircraft is flying economic speed =>speed is almost constant) or a selected speed=>speed is constant.an aircraft doesn't have any limit on GS,so you could easily pass 1000km/u GS,while you're still flying a fixed mach number (IAS).TAS can change due to temperature differences,but will also be nearly constantalso remember it's GS that is responsible for your ETA en EET=>if you have a big tailwind,your GS will increase and so you're gonna be less longer in the air

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Guest rmce

Seba,Brilliant answer.So, it does not matter what speed you are travelling at groundspeed wise, it may never reach Mach.Yeah.Got it.What?I am not denying your expertise in this in any way, its just that I am stupid!

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Guest

All speed indications for passengers is Ground Speed. Indicated at FL370 would be in the mid 200's so they wouldn't want to use that. TAS also would not tell them how fast they are covering the terrain.747-400 cruises around M.90, the Citation X CLIMBS at M .80 and will cruise M.90 or so.Of course it is illegal to fly Mach 1.0 over continental US anyway so the question is moot.We had a moving map screen in the cabin of our Gulfstream where we could let the pax know where we were, ground speed, eta, etc.Kind of pricey at the time but at that time (80's) a skyphone for a jet was about $5,000

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